1. Field of the Invention
The present invention relates to a speed control apparatus for a vehicle, and particularly to such a speed control apparatus which automatically decelerates the vehicle (performs deceleration control) when the vehicle passes through a curve of a road present ahead of the vehicle.
2. Description of the Related Art
As shown in FIG. 9, in an ordinary road, each single curve is composed of an approaching transition curve section, a constant curvature degree section, and a departing transition curve section, in this sequence, from a curve start point (curve entrance) toward a curve end point (curve exit). Each of the transition curves is composed of, for example, a clothoid curve. The transition curve sections are provided in order to enable the vehicle to smoothly pass through the curve through a driver's operation of gradually rotating a steering wheel and then gradually returning the steering wheel to its neutral position, without requiring the driver to rapidly rotate the steering wheel.
Japanese Patent No. 3385812 discloses a speed control apparatus for a vehicle. In the apparatus disclosed in this patent, in order to cause a vehicle to enter a curve at a proper vehicle speed, there are obtained a deceleration distance required for decelerating the vehicle before entering the curve, and a remaining distance, which is the distance between the present position of the vehicle and the curve start point. When the decreasing remaining distance becomes equal to the deceleration distance, deceleration control is started. When the remaining distance becomes zero and the vehicle passes through the curve start point, the deceleration control is ended. That is, the deceleration control is executed so that a vehicle speed appropriate for traveling along the curve is attained at the curve start point.
In an apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2006-331000, a vehicle speed proper for traveling along a curve is determined on the basis of the radius of the curve. The determined proper vehicle speed is then adjusted on the basis of the road length of a constant curvature degree section, or the lengths of transition curve sections (sections each composed of a clothoid curve).
In an apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2009-32031, the following processing is performed in order to obtain a point at which the curvature starts to change (i.e., a curve start point). First, from a curve of a road on which a vehicle travels, a constant curvature degree section is extracted or identified, and the start point and curvature of the constant curvature degree section of the curve are acquired. Identification of this constant curvature degree section is performed on the basis of information regarding actual operation of the vehicle (e.g., a state in which a constant steering angle is maintained continues over a predetermined period of time or more). Subsequently, on the basis of the curvature of the constant curvature degree section, information regarding a transition curve section (a section composed of a clothoid curve), which is connected to the constant curvature degree section, is acquired. The curve start point is acquired on the basis of the information indicating the transition curve section and the start point of the constant curvature degree section. Subsequently, on the basis of the acquired curve start point, suspension control is performed for adjusting the characteristics of the suspensions such that they become suitable for the curve.
That is, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-32031, the curve start point is acquired on the basis of the information regarding the actual operation of the vehicle. Therefore, in a curve along which the vehicle travels for the first time, only the processing of acquiring the curve start point is performed, and suspension control cannot be executed. When the vehicle travels along the curve for a second or subsequent time, the suspension control is executed, while the acquired curve start point is used as a reference.
Here, a case where a curve has a long approaching transition curve section is assumed. In this case, if, as in the apparatus disclosed in Japanese Patent No. 3385812, deceleration control is performed, with the curve start point used as a reference, such that deceleration ends at the curve start point, the deceleration control starts and ends earlier than a driver expects. Therefore, in some cases, the driver feels an unnatural sensation.